The development of Bragg grating reflectors within single mode fibers that are stable and highly selective in wavelength and the demonstration that erbium doped fiber amplifiers (EDFA) are capable of broadband amplification of signals at different, closely space wavelengths, has caused those in the art to consider fabricating a wavelength division multiplex, fiber optic transmission system. However, in order to fabricate such a transmission system, it becomes readily apparent that there is a need for a device for use in adding or dropping a light signal at a predetermined center wavelength to or from such a fiber optic transmission system without disturbing other signals at other wavelengths.
The following describes various devices in the prior art which are candidates for use in fabricating the required add/drop device. A publication entitled "All-Fibre Narrowband Reflection Gratings at 1500 nm" by R. Kashyap, J. R. Armitage, R. Wyatt, S. T. Davey, and D. L. Williams, published in Electronics Lett., vol. 26, 1990, pp. 730-732, discloses a 50%.times.50%, 2 by 2 fiber coupler which is used to couple signal input at all wavelengths and to extract Bragg reflected light. The device and its method of use are disadvantageous in that there is a loss of at least 75% of the reflected light intensity.
A publication entitled "Formation of Bragg gratings in optical fibers by a transverse holographic method" by G. Meltz, W. W. Morey, and W. H. Glenn, published in Optics Lett., vol. 14, 1989, pp. 823-825, discloses the use of a beam splitter to couple broadband light to a Bragg grating. The device and its method of use are disadvantageous in that the beam splitter attenuates both transmitted light and, even more so, reflected light.
A publication entitled "Intermodal coupler using permanently photoinduced grating in two-mode optical fibre" by H. G. Park and B. Y. Kim, published in Electronics Lett., vol. 25, 1989, pp. 1590-1591, discloses a device in which mode coupler gratings are formed by photorefraction in slightly multimode elliptical fibers. The device is disadvantageous in that the grating periodicity is very large and, as a result, the device is not suitable for use in fabricating efficient wavelength division multiplex components.
A publication entitled "Narrow-Bandwidth Optical Waveguide Transmission Filters" by K. D. Hill, D. C. Johnson, F. Bilodeau, and S. Faucher, published in Electronics Lett., vol. 23, 1987, pp. 465-466, discloses a Sagnac loop reflector device which consists of: (a) a twin core fiber at the input and output of the loop and (b) a Bragg grating in the loop which is used to isolate a narrow band of wavelengths. This device and its method of use are disadvantageous in that a beam splitter or coupler is required to transmit the other wavelengths and this causes light loss. Also, for proper functioning, optic path lengths in the loop have to be controlled to fractions of a wavelength and this is difficult to achieve.
In light of the above, there is a need in the art for a device for use in adding or dropping light signals at predetermined center wavelengths to or from a wavelength division multiplex, fiber optic transmission system which carries signals at other wavelengths, which device overcomes the above-described disadvantages. Further, there is a need for such a device for use in adding or dropping a light signal with minimum loss to the light signal being added or dropped and to light signals at other wavelengths already on the transmission system. Still further, there is a need for such a device for use in adding or dropping which can add or drop a predetermined fraction of the light signal. Yet still further, there is a need in the art for such a device for use in adding or dropping wherein the center wavelength of the light signal which is added or dropped is tunable.
In addition to the above, it is well known in the art that dispersion arises in an optical fiber communication system as a result of a change in group index of refraction over a wavelength interval which contains information carried on an optical fiber line which comprises the optical fiber communication system. For example, the zero dispersion wavelength for silica based fibers is approximately 1.3 microns. Thus, for radiation in a band of wavelengths shorter than 1.3 microns, the long wavelength radiation in the band arrives at the end of a length of fiber sooner than radiation at the short wavelengths in the band. Similarly, for silica based fibers, for wavelengths longer than 1.3 microns, radiation at the shorter wavelengths in the interval of wavelengths carrying the information arrive sooner than radiation at the longer wavelengths. Thus, dispersion compensation is required to transmit a signal with high fidelity if the carrier wavelength is other than approximately 1.3 microns. In particular, dispersion compensation is required for optimum transmission if radiation at 1.5 microns is used with silica fibers to take advantage of the use of, for example, erbium doped fiber amplifiers operating substantially at this wavelength.
It is well known in the art that chirped gratings can be used to provide such dispersion compensation. However, prior art investigations into the use of chirped gratings entail fabricating a system wherein output from a single mode core fiber is fed to a chirped grating through a 2 by 2, biconical, tapered coupler. Then, the output from the chirped grating is fed back through the 2 by 2 coupler to an output port of the coupler. One result of this system configuration is a minimum loss of 6 dB in radiation output therefrom due to the radiation passing through the coupler twice.
In light of the above, there is a need in the art for a device for use in providing dispersion compensation without large loss.